Direct fuel injection combined with charge stratification represents a must for two-stroke S.I. engines, since it prevents fuel loss from the exhaust port and incomplete combustion or misfire at light loads. The most difficult aims are keeping stable stratification when engine operating conditions change and, at very light loads, avoiding excessive dilution and spreading of fuel vapour in consequence of burned gas expansion. Two new-concept engine designs are proposed in this paper. In both cases shapes of piston and head, together with scavenging-duct orientation have been optimised to obtain stable in-cylinder flow field features (independently of engine speed) and proper fuel distribution at ignition time. Computational Fluid Dynamics (CFD) predictions at different loads and speeds are reported and discussed.

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